Electrical generators are used in a wide variety of applications. Typically, an individual electrical generator operates in a stand-by mode wherein the electrical power provided by a utility is monitored such that if the commercial electrical power from the utility fails, the engine of the electrical generator is automatically started causing the alternator to generate electrical power. When the electrical output generated by the alternator reaches a predetermined voltage and frequency desired by the customer, a transfer switch transfers the load imposed by the customer from the commercial power lines to the electrical generator. As is known, most residential electric equipment in the United States is designed to be used in connection with an electrical supply having a fixed frequency, namely, sixty (60) hertz (Hz).
Typically, electrical generators utilize a single driving engine coupled to a generator or alternator through a common shaft. Upon actuation of the engine, the crankshaft rotates the common shaft so as to drive the alternator that, in turn, generates electrical power. The frequency of the output of most prior electrical generators depends on a fixed, operating speed of the engine. Typically, the predetermined operating speed of an engine for a two-pole, stand-by electrical generator is approximately 3600 revolutions per minute to produce the rated frequency for which the unit is designed.
It is desirable to maintain the predetermined operating speed of the engine and, therefore, maintain the rated frequency of the generator output. Changes in the magnitude of the load applied to the generator will cause fluctuations in the engine speed and resultant fluctuations in the output voltage and frequency. To minimize the fluctuations in the output voltage, a generator may utilize an automatic voltage regulator (AVR). The AVR receives a signal, or signals, from a sensor, or sensors, connected to the output of the generator which correspond to the current and/or voltage output from the generator. The AVR then regulates the current supplied to the rotor of the alternator to help maintain a constant output voltage at the load.
In order to provide the regulated current to the rotor, the AVR converts power received at an input to the regulated current. Historically, the AVR has been configured to receive power from one of three sources. According to a first option, the AVR may receive power from a battery. Alternatively, a permanent magnet (PM) alternator may be coupled to the rotor, and a stator winding from the PM alternator may provide power to the AVR. As still another option, generator systems utilize a pair of auxiliary windings, where both windings are used to supply power to the AVR.
Each of the afore-mentioned configurations to provide power to the AVR has certain disadvantages. Using a battery requires a system to keep the battery charged. Adding an additional PM alternator to the rotor requires a longer rotor shaft resulting in additional axial length of the alternator. Both the battery and PM alternator add significant cost to the generator system as well. When auxiliary windings are used, a pair of windings is typically used because the voltage induced in the first winding, coupled to the fundamental component of the voltage generated in the alternator, drops to near zero under certain fault conditions, such as a short circuit. In order for the AVR to continue operating under these fault conditions, a second auxiliary winding is provided that is coupled to another harmonic component of the voltage generated in the alternator. Because both the auxiliary windings are also wound on the stator, the size of the stator and the complexity of the windings are increased. Each auxiliary winding is also susceptible to coupling to undesirable harmonic components that may interfere with the AVR operation.
Therefore, it is a primary object and feature of the present invention to provide an improved method for providing power to an AVR of a generator.
It is another primary object and feature of the present invention to provide a single auxiliary winding on a stator that is coupled to desired harmonic components but rejects undesired harmonic components of the voltage generated by the stator.
In accordance with one embodiment of the present invention, an auxiliary winding for use in an alternator of a generator system is disclosed. The alternator includes a stator having multiple slots, a rotor having an excitation winding, and an airgap defined between the stator and the rotor. The auxiliary winding includes multiple turns of wire, where each turn is wound in a first direction in a first slot of the stator and in a second direction in a second slot of the stator. A distribution function defines a number of turns of wire that are present in each of the slots of the stator. The distribution function is defined to couple the auxiliary winding to a fundamental component and a desired spatial harmonic component, selected from multiple spatial harmonic components, of a magnetic flux generated in the airgap of the alternator, and the distribution function is defined to minimize coupling of the auxiliary winding to other spatial harmonic components of the magnetic flux other than the desired spatial harmonic component. The desired spatial harmonic component may be the third harmonic.
According to another aspect of the invention, the distribution function includes a first distribution component configured to couple the auxiliary winding to the fundamental harmonic component of the magnetic flux and a second distribution component configured to couple the auxiliary winding to the a desired spatial harmonic component of the magnetic flux. The first distribution component defines a magnitude corresponding to a portion of the turns of wire in each slot for coupling the auxiliary winding to the fundamental component of the magnetic flux and a sinusoidal function which corresponds to an angular position and a number of pole pairs present in the stator. The second distribution component defines a magnitude corresponding to a portion of the turns of wire in each slot for coupling the auxiliary winding to the desired spatial harmonic component of the magnetic flux and a sinusoidal function which corresponds to the angular position, the number of pole pairs present in the stator, and the desired spatial harmonic.
According to another embodiment of the invention, an alternator configured to be driven by an engine in an engine-driven generator system is disclosed. The alternator includes a stator, a rotor, and an AVR. The stator includes multiple slots, a main winding distributed in the plurality of slots, and an auxiliary winding also distributed in the plurality of slots. The auxiliary winding is distributed according to a distribution function which couples the auxiliary winding to a fundamental component and a desired spatial harmonic component of a magnetic flux generated in an airgap of the alternator. The distribution function also minimizes coupling of the auxiliary winding to other spatial harmonic components of the magnetic flux. The rotor is rotatably mounted within the stator and driven by the engine and includes an excitation winding configured to conduct a current which establishes the magnetic flux in the airgap. The AVR controls the current in the rotor as a function of at least one of a current and a voltage present on the main winding.
According to another embodiment of the invention, a method of providing power to an excitation winding on a rotor of an alternator in a generator system is disclosed. The generator system includes a single auxiliary winding and a main winding each wound on a stator of the alternator, and the stator is separated from the rotor by an airgap. The auxiliary winding receives power corresponding to a current conducted by the excitation winding and is wound on the stator to couple the auxiliary winding to a fundamental component and a desired spatial harmonic component of a magnetic flux generated in the airgap of the alternator and to minimize coupling of the auxiliary winding to other spatial harmonic components of the magnetic flux. Power is transmitted from the auxiliary winding to an automatic voltage regulator (AVR) and controlled by the AVR to supply the excitation winding as a function of the output voltage of the main winding.